In recent years, development of environmentally-friendly products has continued to advance in the coating material industry, as environmental pollution increases and society shifts toward greater recycling with the aim of reducing the burden on the environment. In light of this situation, various coating agents employing materials with biodegrading action have been proposed, since they naturally do not contain significant amounts of volatile organic compounds (VOC), such as toluene and xylene, that can pollute the indoor environment, but, also do not cause soil contamination after disposal.
In Japanese Unexamined Patent Publication No. 2004-224887, for example, there is disclosed a curable starch composition which is a mixture of starch and a curing agent having a functional group that reacts in a complementary manner with at least one hydroxyl group in the starch molecule. It is stated that the curable starch composition can be used in a form dispersed in an aqueous solvent. However, it is not easy to achieve stable dispersion of starch in aqueous media.
Japanese Unexamined Patent Publication No. 2001-40267 discloses a releasable or moisture-proof coating composition comprising a specific starch ester, a hydrophobic plasticizer and water. However, the starch ester used here is hydrophobic and therefore poorly dispersible in water, and the finished appearance of the formed coating film at ordinary temperature is poor.
Also, Japanese Unexamined Patent Publication No. 2006-52338 discloses a modified starch-containing resin aqueous dispersion comprising a (co)polymer of modified starch and a polymerizable unsaturated monomer as constituent components. Although the storage stability of the aqueous dispersion is significantly improved due to the aid of the copolymer resin, long-term storage nevertheless often results in settling of the dispersion particles and increasing viscosity of the dispersion, thus limiting the freedom of the coating material design.
Japanese Unexamined Patent Publication No. 8-239402, on the other hand, discloses an invention relating to graft starch obtained by indirectly grafting a starch resin and acrylic resin through a polyisocyanate, and graft starch obtained by radical graft polymerization of an unsaturated monomer onto starch or modified starch.
As an example of combining starch with another plant-derived resin, Japanese Unexamined Patent Publication No. 6-207047 discloses an invention wherein a polymer blend comprising a combination of starch or modified starch with a cellulose derivative is used as a molding material. There are also disclosed inventions relating to resin compositions employing starch-based resins as water-absorbing resins.
This prior patent literature clearly demonstrates that starch-based resins obtained by combining or grafting different polymers are themselves known. However, all of these techniques assume that the uses of the starch-based resins are for adhesives, structural materials, injection molding materials, sheets or the like, whereas virtually no uses as coating materials have been disclosed.
Also, Japanese Unexamined Patent Publication No. 2006-282960 discloses a curable starch composition comprising starch, a polyisocyanate curing agent, a plant-derived resin other than starch, a metal complex, and a blocking agent selected from among β-diketones, acetoacetic acid esters, malonic acid esters, ketones with β-hydroxyl groups, aldehydes with β-hydroxyl groups and esters with β-hydroxyl groups.
However, a method of forming coating films using these starch-based coating materials, allowing formation of coating films with excellent finished appearance, pencil hardness, mar resistance, adhesion, weather resistance, alkali resistance and solvent resistance on substrates, has not existed. In addition, because most of the coating materials used are organic solvent-based coating materials, the usage of large amounts of solvents has been a problem.